(1) Field of the Invention
This invention has particular utility in the field of ink jet printing and, more particularly, to a multi-layered ceramic electrostatic drop sensor with sensor diagnostic feature.
(2) Description of the Prior Art
In ink jet printers of the type where an ink jet head traverses along a print line on a paper at a velocity which varies as a function of time, it is necessary to provide on-the-fly determination of the correct lead distance over which to release ink drops so as to cause accurate placement of the drops on the paper by simultaneously measuring the head transport induced stream velocity V.sub.n and quickly performing the calculation for the lead time d based upon a measured value of drop flight time T.sub.f.
The relationship between velocity components V.sub.n, V.sub.s, and V.sub.r is shown in a publication by H. W. Johnson, "Drop Velocity Compensation In Moving Head Ink Jet Printers", IBM Technical Disclosure Bulletin, Vol. 20, No. 11B, April 1978, pp. 4920-21, along with a diagram which shows the relationship between s, d, and r where
V.sub.h =head transport velocity PA0 V.sub.s =pump pressure induced stream velocity PA0 V.sub.r =resultant drop velocity PA0 d=head displacement during drop flight or horizontal component of drop displacement during flight PA0 s=distance from drop break-off point to paper PA0 r=resultant drop displacement
Since the corresponding angles of the triangles are equal, the triangles are similar, and EQU d/s=V.sub.h /V.sub.s EQU d=SV.sub.h /V.sub.s
But s/V.sub.s is the drop flight time, T.sub.f, and (neglecting aerodynamic and other effects) EQU d=V.sub.h T.sub.f
The significance of d is that it is the component of drop displacement that is parallel to the paper and thus represents the amount of "lead" required when releasing a drop in order to place it at a desired location on the paper, or recording medium.
The flight time, T.sub.f, can be measured both statically and dynamically. The static measurement is taken with the head stationary and aligned at a service station with a flight time sensor off to one side of the recording medium, as is suggested by U.S. Pat. No. 3,977,010 (Erickson, et al). U.S. Pat. No. 4,176,363 (Kasahara) describes an ink jet printing apparatus, and includes an illustration of position C where certain tests may be performed on the head 12. Kasahara describes, therefore, the positioning of a head at a "service station" as is referenced in Erickson, et al.
Various sensor structures and circuitry for measuring flight time and other ink jet drop stream characteristics have been suggested in the prior art. These include the following:
U.S. Pat. No. 3,852,768 (Carmichael, et al) describes charge detection for ink jet printers. An assembly of laminar elements including a sensor element, an inner shield, and an outer shield has an aperture through which ink drops pass. The drops passing through the aperture are capacitively coupled to the sensor for generating charges thereon in timed relation to passage of the drops. A loss in signal output from the sensor indicates stream failure. The laminar elements comprise alternate sheets of copper and Mylar* FNT (*Registered Trademark of Dupont).
U.S. Pat. No. 3,886,564 (Naylor, et al) describes a deflection sensor for ink jet printers involving differential sensing of signals developed from charged drops, and having utility in sensing, inter alia, drop velocity and ink stream failure.
U.S. Pat. No. 3,977,010 (Erickson, et al) describes a dual sensor for multi-nozzle ink jet, which selectively measures flight time or stream alignment of electrostatically charged drops. During the test cycle, the head to be tested is moved to a service station off to one side of the recording medium, and selector 135 operated to select the sum (flight time) measurement or the difference (alignment) measurement (see FIG. 10). Erickson further teaches the use of flight time measurements (where flight time is the inverse of velocity) to adjust the pressure or viscosity of the ink, and for indicating a charge electrode failure or improper synchronization of the charge signal in the head.
U.S. Pat. No. 4,121,223 (Omori, et al) describes an ink sensor including a copper/insulator laminated structure mounted to the ink gutter for detecting error in the phase between emission of ink droplets out of a nozzle and the charging thereof.
U.S. Pat. No. 4,101,906 (Dahlstrom, et al) describes a charge electrode assembly for an ink jet printer including a nonconductive ceramic with grooves into which a passive noble metal, such as platinum or rhodium, is sputtered to form a conductive layer. Such a structure is found to be resistant to degradation by the impingement of pressurized ink jet streams or electrochemical attack.
U.S. Pat. No. 4,158,204 (Kuhn, et al) describes a time correction system for multi-nozzle ink jet printer. A sensor positioned downstream from a nozzle in the path of the ink drops is used to determine the flight time, which may vary due to nozzle imperfections, chearances, accumulations and deposits of ink. The calculated flight time is used to control the time at which information signals are applied to each of a plurality of charge electrodes during printing.
U.S. Pat. No. 3,953,860 (Fujimoto, et al) describes a charge amplitude detection apparatus for an ink jet printer. The amplitude of charge on phase detecting drops is detected by electrostatic induction in a panel or strip shaped detection electrode adjacent the wake of the ink drops.
U.S. Pat. No. 3,836,912 (Ghougasian, et al) describes a drop charge sensing apparatus for an ink jet printing system. The sensing element includes a conductive member placed downstream from a charging station proximate to, but in non-impinging relationship with the droplet stream. The electrostatic charge on each drop is sensed by the inductive charge sensing member, and used to control the sychronization of ink droplet formation and the application of video charging signals to the ink droplet stream.
U.S. Pat. Nos. 4,167,013 (Hoskins, et al) and 4,167,014 (Darling, et al) describe circuitry for perfecting ink drop printing at nonlinear, or varying, carrier velocity. In each, it is assumed that the drop velocity is a contstant, and circuitry is provided for calculating the lead time for a given print position for varying print head velocities.
In the above references, apparatus is provided for sensing the charge on charged ink drops. Such sensors deal with very weak field intensities and therefore with very small signal currents. Consequently, the physical environment of the drop sensor, including the wetness and contaminants of the ink, tends to degrade the operation of the sensor and result in sensor failure. Further, errors and failures can occur in the electronic circuitry associated with the sensor. Consequently, it is desirable and advantageous to provide a sensor which, without human intervention, is capable of measuring drop flight time, while also detecting failure in the ink drop forming head, failure in the sensor antenna plates, and failure in the sensor electronics.